Laser sources used in detonators must be robust, space saving and economical, especially for military or astronautical applications. They are therefore either Nd-YAG-solid lasers (for military applications) with a power density of about 3 MW·cm−2 or laser diodes with, generally, 1 W power output (for astronautical applications) and a power density of about 20 KW·cm−2, which is too low for direct initiation of the secondary explosive detonation, for which a power density of about 1 GW·cm−2 is required.
These power densities lead, however, to a temperature increase of the secondary explosive in the first detonator stage up to the achievement of the self-sustaining decomposition temperature at which subsequently a very violent breakdown reaction takes place by which the secondary explosive detonation in the second stage is initiated (depending on the detonator configuration and the characteristics of the secondary explosives used) either by a deflagration-detonation transition process or a percussion-detonation transition process. However, since the secondary explosives do not absorb the light in the near infrared range emitted by the laser sources, the energetic igniter charge in the first detonator stage is a mixture of secondary explosive and soot powder which is used as optical doping material (absorbs the radiation emitted by the laser sources and transfers the required heat energy for the achievement of the critical temperature of the secondary explosive).
The effectiveness of soot however decreases strongly in applications in which the detonator is exposed to extreme climatic conditions. For the validation of a detonator for such an application, experiments must be conducted emulating a temperature variation stress according to the requirements of this application. In the field of astronautics, such a temperature variation stress includes, for example, a temperature increase to 100° C. during five hours as well as a subsequent cooling down to room temperature. When a laser diode is used as the laser source, ignition of the secondary explosive mixture with 1 percent by weight (wt. %) soot no longer occurs after such a temperature variation stress even with a maximum diode power of 1 W, although a power of 0.1 W is normally sufficient for ignition of the detonator.
A first solution to the problem of providing the required high power laser source for ignition of a detonator under such difficult climatic conditions is described in French Patent FR 2 831 659, according to which a pyrotechnic redox mixture is placed in the first detonator stage between the secondary explosive and the optical focusing interface which absorbs light in the infrared range and initiates a redox reaction in which the required heat energy for ignition of the secondary explosive is released. The pyrotechnic mixture used (ZPP) is however generally very sensitive to friction and electrostatic discharges.
Furthermore, for a reliable ignition of the pyrotechnical redox mixture in optical initiators with the use of a laser diode (especially 1-W laser diode) as laser source, pyrotechnic mixtures must be used, the reducing agent of which has a very fine particle size (typically between 1 and 2 μM). However, because of this particle size, the pyrotechnical redox mixture is extremely sensitive to friction and electrostatic discharges, which leads to dangerous manufacture and handling.
It is now an object of the present invention to ignite an optical igniter (detonator or initiator) with a laser source of low power and to provide a solution for the above mentioned problem inherent with igniters of the last generation.